Ultrasonic imaging, also called echography or B-mode ("brightness mode") ultrasound, involves use of an ultrasonic transducer which repeatedly emits pulses of high-frequency sound and receives the resulting echo signals. A focused beam of sound is normally used, and various means are employed to sweep this beam repeatedly through a range of directions. Electronic processing of the received echo signals, synchronized to the movement of the beam, results in formation of a video image (normally cross-sectional) of structures (such as human tissue) in the beam's path.
For clarity of exposition, the following discussion refers specifically to the medical diagnostic imaging situation in which the target of the ultrasonic imaging system is tissue in a living human patient. It should be realized that ultrasonic scanning is also used in other applications including non-human (veterinary or in vitro tissue sample) biological imaging, and also non-biological imaging applications (non-destructive materials testing), and that the present invention applies without limitation to all such ultrasonic imaging applications.
In ultrasonic scanners for medical diagnostic imaging, the transducer and beam-sweeping components are normally assembled in the form of a hand-held probe connected to the rest of the imaging system by means of a cable. In the following, the term "probe" will be used generically to refer to the transducer and beam-scanning assembly, though it should be realized that not all ultrasonic scanning systems will have or need a physically distinct probe component.
Because the sound frequencies used in ultrasonic imaging are effectively blocked by air, it is necessary to acoustically couple the transducer to the target under investigation via one or more acoustically conductive media. Coupling media must be chosen carefully, to ensure that the effects of sound reflection and refraction occurring at media interfaces do not unduly compromise the imaging capabilities of the system. In practice, the problem of coupling is addressed in one of three ways:
1. Liquid bath. The transducer and target are immersed in a liquid coupling medium such as water or sterile saline solution. This technique is rarely used in clinical scanning applications. PA1 2. Sealed transducer chamber. The transducer is sealed within a liquid-filled chamber within the probe, with a solid "acoustic window" through which the imaging beam can pass. PA1 3. Acoustic coupling gel. Several water-based gel media are now on the market, which provide a convenient means to couple a probe's acoustic window to the surface of target tissue.
Many techniques for sweeping the sound beam are known, but as at the date of filing of this application, there are two main approaches which are in common use. In the mechanical sector-scan approach, a transducer is mechanically oscillated about a pivot, causing the sound beam to sweep through a sector. Acoustic coupling to target tissue may be achieved either by method 1 above, or more commonly, a combination of methods 2 and 3. In the electronic transducer array approach, a fixed array of multiple transducer elements is used, and the sound beam is formed, focused, and swept entirely electronically. Because there is no transducer movement, coupling method 2 above is unnecessary; method 3 alone is sufficient.
The electronic transducer array approach generally provides greater speed, precision and repeatability of sound beam motion than the mechanical sector-scan approach, and hence today the array approach is widely used. However, the array approach, which requires one signal processing channel (e.g. impedance matching and filtering, signal conditioning and amplification, and sometimes digitization) per array element, is inherently more expensive than the mechanical sector scanning approach, and so the mechanical sector scanning approach is still used. In the field of ophthalmology, for example, mechanical sector scanners are still used exclusively. There thus remains a need to advance the state of the art with respect to the mechanical sector scan approach.
So-called annular array transducers, which consist of a small number of concentric ring-shaped transduction elements, may be used in place of a single-element transducer in a mechanical sector scanning apparatus. These devices permit electronic control of sound beam focusing, but not direction. Because the number of transduction elements is small (e.g. three to eight), the cost premium associated with annular array transducers is slight compared with that of a fall electronic array beam sweeping system.
In this application, the term "transducer" refers equally to either a single element transducer or an annular array transducer.
Ultrasonic scanning and image formation can occur quite rapidly, which makes possible dynamic imaging in the presence of motion, e.g. of the living fetus in utero, of the tissues in the eye, or in the course of interventional medical procedures such as catheterization or laparoscopic surgery. The fidelity with which continuous motion can be represented in the live ultrasonic image depends primarily on how rapidly the sound beam can be scanned across the field of view, and on how precisely and repeatably the scanning motion can be controlled.
Purely mechanical means of oscillation, with or without use of magnetic elements on some of the moving members to transfer force across a gap, exhibits a number of disadvantages. The mechanical drive assembly adds substantially to the size and weight of the imaging probe, and to achieve precisely controlled motion it is necessary to augment the mechanical system with a positional sensor and to employ closed-loop feedback control methods in the control system. Addition of the positional sensor adds yet further to the size, weight, and complexity of the probe, while the need for closed loop feedback control adds to the complexity of the control system.
However, motive force may also be applied by means of magnetic forces established between fixed electromagnets, energized with time-varying electric current, and a single permanent magnet affixed to a rotating transducer assembly. Generally, in all of these cases the magnetic oscillator mechanism follows the general principle of a galvanometer, i.e., a magnetic field is established in a fixed stator member, with the magnetization vector substantially at right angles to that of a permanent magnet affixed to a pivoted rocker member. The magnet in the rocker is thus subjected to magnetic torque forces which cause it, and the rocker, to tilt or rotate so as to reduce the angle between the two magnetic vectors. Periodically reversing the direction of the stator field, by passing an alternating current through the one or more electromagnet coils in the stator assembly, thus results in a periodically reversing torque applied to the rocker, and in consequence causes the rocker to oscillate.
In practice, such galvanometer-like mechanisms cannot transfer appreciable amounts of mechanical energy, because of the inverse square relationship between magnetic force and distance. As the rocker magnet tilts away from the center position, it moves further from the stator magnet poles, dramatically reducing the torque efficiency of the system and making it harder to apply the reverse torque required to tilt the rocker back in the opposite direction. Such a system is rather like a heavy weight balanced atop a rod held in the hand; provided the rod does not tip very far from the vertical, small movements of the hand suffice to keep it stably aloft, but the range of controllable motion is very small.
The present invention solves the torque efficiency problems by using a system of many digitally switched stator electromagnet coils, which permit periodic alteration of the magnetic field vector while maintaining a small and little-varying gap between the currently active stator poles and a permanent magnet in the rocker, and furthermore solves the problem of stability at higher operating speeds by providing additional permanent magnets affixed to the stator near the extremes of the rocker's oscillatory motion, oriented so as to repel the permanent magnet in the rocker, causing it to spring back in the opposite direction.